An international team of astronomers has found the smallest Earth-like planet yet outside our Solar System.

The new planet has five times the Earth's mass and can be found about 25,000 light-years away in the Milky Way, orbiting a red dwarf star.

The discovery, reported in the journal Nature, was made using a method called microlensing, which can detect far-off planets with an Earth-like mass.

The planet's cold temperatures make the chance of finding life very unlikely.

The planet, which goes by the name OGLE-2005-BLG-390Lb, takes about 10 years to orbit its parent star, a red dwarf which is cooler and smaller than Earth's Sun.

It is in the same galaxy as Earth, the Milky Way, but is found closer to the galactic centre.

Albert Einstein

The scientists say it probably has a rocky core and perhaps even a thin atmosphere, but its large orbit and cool parent star mean it is a very cold world.

Predicted surface temperatures are minus 220 degrees Celsius (-364F), meaning that its surface is likely to be layer of frozen liquid. It may therefore resemble a more massive version of Pluto.

"This is very exciting and important," said Professor Michael Bode from Liverpool John Moores University, a principal investigator for the RoboNet project which collaborated on this research.

"This is the most Earth-like planet we have discovered to date, in terms of its mass and the distance from its parent star," he told BBC News.

"Most of the other planets that have been discovered are either much more massive, much hotter or both."

The microlensing technique used to find this planet was first predicted by Albert Einstein in 1912.

Microlensing occurs when a massive object in space, like a star, crosses in front of another more distant star.

As it passes, the gravity from the foreground object bends the light coming from the background star, temporarily making it look brighter. This usually lasts for about a month.

How can we prove there is life on a distant planet when we have problems seeing if there is life on Mars?

Dr Martin Dominik, University of St Andrews

If the foreground star has a planet orbiting it, it will distort the light even more, and will make the star behind it look even brighter. But this effect lasts for a much shorter period, giving astronomers just hours or days to detect it.

Dr Martin Dominik from the University of St Andrews is a co-leader of the PLANET collaboration, one of the microlensing networks used to detect the new planet.

"We first saw the usual brightening reaching a peak magnification on 31 July 2005. On 10 August, however, there was a small 'flash' lasting about half a day," he said.

"By succeeding in catching this anomaly with two of the telescopes of our network and with careful monitoring, we were able to conclude that the lens star is accompanied by a low-mass planet."

Life on Earth

The discovery was the joint effort of three microlensing campaigns, PLANET/RoboNet, OGLE and MOA, and involved researchers from 12 countries.

So far, about 160 planets have been found outside our Solar System, but only three of them have been located using the microlensing technique.

Recent simulations of planet formation suggest that bodies with an Earth-like mass are abundant. Scientists are attempting to discover more new worlds using this technique and are looking for ways to refine it further.

Dr Nicholas Rattenbury, from Jodrell Bank Observatory in Cheshire, a member of the MOA microlensing collaboration, points out: "We could take this research forward by building a network of bigger telescopes around the world to make us more efficient at detecting these Earth-like planets."

If planets are found with conditions similar to our own planet, then the next step would be to begin the search for life, but this might not prove easy.

"To prove there is life on a far-off planet would be difficult," Dr Dominik told the BBC News website.

"How can we prove there is life on a distant planet when we have problems seeing if there is life on Mars?"